Re: [PATCH] xfs: handle array index overrun in xfs_dir2_leaf_readbuf()

Darrick J. Wong
2017-04-19T16:09:00Z

On Wed, Apr 19, 2017 at 09:40:20AM -0500, Eric Sandeen wrote:
> On 4/19/17 7:32 AM, Brian Foster wrote:
> > On Tue, Apr 18, 2017 at 05:29:27PM -0700, Darrick J. Wong wrote:
> >> On Tue, Apr 18, 2017 at 07:14:03PM -0500, Eric Sandeen wrote:
> >>> On 4/18/17 7:09 PM, Darrick J. Wong wrote:
> >>>> On Tue, Apr 18, 2017 at 12:55:44PM -0400, Brian Foster wrote:
> >>>>> On Mon, Apr 17, 2017 at 05:12:36PM -0500, Eric Sandeen wrote:
> >>>>>> On 4/17/17 3:57 PM, Brian Foster wrote:
> >>>>>>> On Thu, Apr 13, 2017 at 01:45:43PM -0500, Eric Sandeen wrote:
> >>>>> ...
> >>>>>>>
> >>>>>>> This fix seems fine to me, but I'm wondering if this code may have
> >>>>>>> issues with other kinds of misalignment between the directory blocks and
> >>>>>>> underlying bmap extents as well. For example, what happens if we end up
> >>>>>>> with something like the following on an 8k dir fsb fs?
> >>>>>>>
> >>>>>>> 0:[0,xxx,3,0]
> >>>>>>> 1:[3,xxx,1,0]
> >>>>>>>
> >>>>>>> ... or ...
> >>>>>>>
> >>>>>>> 0:[0,xxx,3,0]
> >>>>>>> 1:[3,xxx,3,0]
> >>>>>>
> >>>>>> Well, as far as that goes it won't be an issue; for 8k dir block sizes
> >>>>>> we will allocate an extent map with room for 10 extents, and we'll go
> >>>>>> well beyond the above extents which cross directory block boundaries.
> >>>>>>
> >>>>>>> ...
> >>>>>>> N:[...]
> >>>>>>>
> >>>>>>> Am I following correctly that we may end up assuming the wrong mapping
> >>>>>>> for the second dir fsb and/or possibly skipping blocks?
> >>>>>>
> >>>>>> As far as I can tell, this code is only managing the read-ahead state
> >>>>>> by looking at these cached extents. We keep track of our position within
> >>>>>> that allocated array of mappings - this bug just stepped off the end
> >>>>>> while doing so.
> >>>>>>
> >>>>>> Stopping at the correct point should keep all of the state consistent
> >>>>>> and correct.
> >>>>>>
> >>>>>> But yeah, it's kind of hairy & hard to read, IMHO.
> >>>>>>
> >>>>>> Also as far as I can tell, we handle such discontiguities correctly,
> >>>>>> other than the bug I found. If you see something that looks suspicious,
> >>>>>> I'm sure I could tweak my test case to craft a specific situation if
> >>>>>> there's something you'd like to see tested...
> >>>>>>
> >>>>>
> >>>>> Background: Eric and I chatted a bit on irc to rectify that what I'm
> >>>>> calling out above is a different issue from what is fixed by this patch.
> >>>>>
> >>>>> Eric,
> >>>>>
> >>>>> I managed to construct a directory that looks like this:
> >>>>>
> >>>>> EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
> >>>>> 0: [0..7]: 88..95 0 (88..95) 8
> >>>>> 1: [8..15]: 80..87 0 (80..87) 8
> >>>>> 2: [16..39]: 168..191 0 (168..191) 24
> >>>>> 3: [40..63]: 5242952..5242975 1 (72..95) 24
> >>>>>
> >>>>> The fs has 8k directory fsbs. Dir fsb offset 0 spans extents 0 and 1,
> >>>>> offset 1 (which corresponds to the 512b range 16-31 above) is covered
> >>>>> completely by extent 2 and dir offset 2 (range 32-47) spans extents 2
> >>>>> and 3. An ls of this directory produces this:
> >>>>>
> >>>>> XFS (dm-3): Metadata corruption detected at xfs_dir3_data_reada_verify+0x42/0x80 [xfs], xfs_dir3_data_reada block 0x500058
> >>>>> XFS (dm-3): Unmount and run xfs_repair
> >>>>> XFS (dm-3): First 64 bytes of corrupted metadata buffer:
> >>>>> ffffbcb901c44000: 00 00 00 00 00 00 00 64 0f 78 78 78 78 78 78 78 .......d.xxxxxxx
> >>>>> ffffbcb901c44010: 78 78 78 78 2e 38 38 36 01 00 00 00 00 00 10 00 xxxx.886........
> >>>>> ffffbcb901c44020: 00 00 00 00 00 00 00 64 0f 78 78 78 78 78 78 78 .......d.xxxxxxx
> >>>>> ffffbcb901c44030: 78 78 78 78 2e 38 38 37 01 00 00 00 00 00 10 20 xxxx.887.......
> >>>>>
> >>>>> ... which is yelling about block 184 (dir fsb 2). The fs is otherwise
> >>>>> clean according to xfs_repair.
> >>>>>
> >>>>> I _think_ something like the appended diff deals with it, but this is
> >>>>> lightly tested only and could definitely use more eyes.
> >>>>>
> >>>>> Brian
> >>>>>
> >>>>> --- 8< ---
> >>>>>
> >>>>> diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> >>>>> index ad9396e..9fa379d 100644
> >>>>> --- a/fs/xfs/xfs_dir2_readdir.c
> >>>>> +++ b/fs/xfs/xfs_dir2_readdir.c
> >>>>> @@ -404,7 +404,8 @@ xfs_dir2_leaf_readbuf(
> >>>>> * Read-ahead a contiguous directory block.
> >>>>> */
> >>>>> if (i > mip->ra_current &&
> >>>>> - map[mip->ra_index].br_blockcount >= geo->fsbcount) {
> >>>>> + (map[mip->ra_index].br_blockcount - mip->ra_offset) >=
> >>>>> + geo->fsbcount) {
> >>>>> xfs_dir3_data_readahead(dp,
> >>>>> map[mip->ra_index].br_startoff + mip->ra_offset,
> >>>>> XFS_FSB_TO_DADDR(dp->i_mount,
> >>>>> @@ -432,7 +433,7 @@ xfs_dir2_leaf_readbuf(
> >>>>> * The rest of this extent but not more than a dir
> >>>>> * block.
> >>>>> */
> >>>>> - length = min_t(int, geo->fsbcount,
> >>>>> + length = min_t(int, geo->fsbcount - j,
> >>>>
> >>>> Looks ok to me to make Eric's bugfix complete.
> >>>
> >>> Brian, thanks for crafting the image to expose this. :)
> >>>
> >>> I've been otherwise occupied today, sorry - technically this fixes a separate
> >>> issue, yes? So 2 patches, 2 bugfixes AFAICT?
> >>
> >> Ok. I don't mind taking two patches since you're right, the first patch
> >> saves us from running off the end of the shadow bmap array, and the
> >> second one fixes bookkeepping problems when a dir extent starts midway
> >> through a large dirblock.
> >>
> >
> > Yeah, these are definitely separate issues and Eric's original patch
> > here is probably higher priority on its own.
> >
> > But I'm not clear on the plan here.. do we want this additional patch or
> > are we planning just to rewrite the readahead bits (or do we want to do
> > both)? I guess having a bugfix patch in the tree is useful from a stable
> > perspective... hm?
> >
> > Brian
>
> Yeah.
>
> There's 20 years of the old code in the field with these bugs - I think 2
> straightforward fixes have value even if we might want to rewrite it in the
> future.

Assuming 4.11 is released on Sunday, I was figuring 4.12 for the two
fixes + backporting to older kernels, and explore the rewrite for 4.13.

--D

>
> -Eric